Integrated circuits are designed to operate in electronic systems, which perform larger system functions. Some integrated circuits, referred to as application specific integrated circuits (ASICs), operate in specific applications. An ASIC can be a device including one or more independent functions, a system-on-a-chip or a combination of independent functions and a system-on-a-chip. The system-on-a-chip can include a core processor, memory, and peripheral storage and logic elements.
Integrated circuits typically include circuit elements clocked by oscillating signals to regulate the operation of the system. In an ASIC, an oscillating signal is brought into the device and distributed throughout the circuit. System operation may come to a halt in the event the signal stops oscillating. A system so impaired can leave system components in unsafe and wasteful states, such as driving a motor at maximum current.
Many systems include a watchdog timer (WDT) circuit to reset the system in the event the microprocessor malfunctions or operates uncontrollably. In a conventional WDT circuit, a counter is clocked by an oscillating signal from an initial value toward a predetermined value. The WDT is serviced by a microprocessor to reconfigure the counter to an initial value. The WDT circuit provides a reset signal to the system in the event the counter reaches the predetermined value before being serviced by the microprocessor. With a WDT, the system is reset in the event the microprocessor operates uncontrollably, such as when the code hangs or the microprocessor becomes unstable after an electro-static discharge (ESD) event. However, the WDT counter needs an oscillating signal to count toward the predetermined value. The counter cannot reach the predetermined value to reset the system after the oscillating signal has stopped.
In one circuit that detects an oscillating signal has stopped, a first timer circuit and a second timer circuit receive the oscillating signal in parallel. One timer circuit detects the oscillating signal has stopped at a high signal level and the other timer circuit detects the oscillating signal has stopped at a low signal level. Each timer circuit includes a capacitor, which is charged and discharged during each cycle of the oscillating signal. After the oscillating signal stops, the capacitor is discharged to a predetermined value and a reset signal is provided.
In another circuit, a one-shot pulse circuit provides a one-shot pulse in response to an oscillating signal. A charge/discharge circuit includes a capacitor, which discharges while the one-shot pulse is high and otherwise charges. A reset signal is generated after the charge voltage on the capacitor exceeds a predetermined value.
The circuit with two timers and the one-shot circuit charge and discharge a capacitor to determine the state of the oscillating signal. The value of the capacitor must be changed to adjust the circuit to oscillating signals of different frequencies. For this reason, the capacitor is usually provided as an external component, which adds cost to the electronic system.